Refractory articles and methods for producing refractory articles

ABSTRACT

Refractory articles and methods for producing refractory articles are provided. One method comprises producing a mix of particulate pitch, ceramic filler and water, spray drying the mix, pressing the spray dried mix to form an intermediate product and heating the intermediate product to product a refractory article comprising graphitisable carbon bond.

The present invention relates to refractory articles and methods forproducing refractory articles, in particular black refractory articles.

During the processing of steel, various refractory items such as ladleshrouds, stoppers, submerged entry nozzles and submerged entry shroudsare used to protect the steel flow from oxidation by air and otherwiseused during steel formation and processing techniques. Such items areusually made of iso-statically pressed carbon bonded alumina-graphite,zirconia graphite and magnesia graphite. These are generally termedBlack refractory articles.

Conventionally, such carbon bonding refractories are produced by mixingphenolic resin with ceramic powder, such as alumina, zirconia ormagnesia, graphite and other additives such as glass frit and metallicpowders, such as silicon and aluminium. Hexamethylene tetraamine is alsoadded to cross link the phenolic resin. High intensity mixing isgenerally required to homogenise such mixes, and the homogenised mixthen passed through a dryer to dry off the excess solvent to producegranules. The granules are then generally pressed into the shape of thedesired article. Following pressing, the articles are placed in an ovenand heated to about 200-250° C. for approximately 24 hours in order totemper the article by converting the phenolic resin from a thermoplasticto a thermostatic material by cross-linking reaction with thehexamethylene tetraamine.

Following these stages, the article is fired at temperatures between700° C. and 1000° C. in order to pyrolysis the resin and form a carbonbond network that gives the article its characteristic refractoryproperties of relatively good resistance to thermal shock, corrosion anderosion from steel flow and good mechanical toughness. However thenature of the carbon bond produced in this way is a glassy carbon whichhas relatively low oxidation resistance and lower thermal shockresistance compared to the graphatisable type of carbon bond usuallyobtained from pyrolyisis of tar pitch, using similar processes.

However, these conventional processes of producing black refractoryproducts suffer from a number of significant disadvantages. Phenolicresin and pitch are both hazardous to the environment and to peopleprocessing the materials. Furfuryl alcohol which is the usual liquidpresent in the phenolic resin is carcinogenic, as is tar pitch. Theprocess for driving off the solvent to produce the granules is difficultto control, and the mix has a limited shelf life. It is also found thatthe quality of finished articles is not consistent and the heating stagefor pyrolysis is very difficult to control due to the evolution offlammable gases. The process is long, taking over 72 hours from start tofinish and can only be processed in batches. The carbon yield ofphenolic resin is low, usually less than 50%.

According to the present invention there is provided a method forproducing a refractory article, the method comprising producing a mix ofparticulate pitch, ceramic filler and water, spray drying the mix,forming the spray dried mix into an intermediate product and heating theintermediate product to produce a refractory article comprisinggraphitisable carbon bond.

Preferably the mix is produced in the form of a slurry and is generallyhomogenous. The particulate pitch used is preferably in powder form anddesirably a high melting carbon bond pre-cursor. The pitch may compriseCARBORES a material supplied by RUTGERS VFT AG of Castrop-Rauxel,Germany.

The ceramic filler used may comprise one or more of alumina, zirconia,magnesia. The mix may further comprise one or more of silicon carbide,anti-oxidant, metallic powder, glass frit, preservative material,stabilising material, anti-bacterial material, tabular alumina, mullite,silica, chromite, clay talc, mica, silicon carbide, silicon nitride,zirconium diboride, titanium diboride, boron metal powder, magnesiummetal powder, aluminium metal powder, boron carbide.

Preferably the mix is spray dried in spray drying apparatus, which maybe of conventional design.

The mix is preferably spray dried to produce granules, which arepreferably free flowing.

The granules are preferably pressed desirably at ambient temperature,preferably in an iso-static press.

The intermediate product is preferably heated to temperatures in theorder of 700° C. to 1000° C., for between approximately 0.1 and 3 hours,desirably to fire the product to produce graphitisable carbon bonds inthe article. Preferably heating is carried out generally in the absenceof air. Alternatively or in addition the intermediate product may beglazed to protect from oxidation during pyrolysis.

The method is preferably carried out continuously and may be automated.

According to a further aspect of the present invention there is provideda method for producing granules for use in the production of refractoryarticles, the method comprising producing a mix of particulate pitch,ceramic filler and water and spray drying the mix.

According to a further aspect of the present invention there is provideda refractory article produced according to the methodology describedabove in any of the preceding nine paragraphs.

The invention further provides granules for use in the production ofrefractory articles, the granules being produced and being substantiallyas described in any of the preceding ten paragraphs.

Preferred embodiments of the present invention will now be described byway of example only.

The invention provides refractory articles and methods for producingrefractory articles. One method comprises producing a mix of particulatepitch, ceramic filler and water, spray drying the mix, pressing thespray dried mix to form an intermediate product and heating theintermediate product to produce a refractory article comprisinggraphitisable carbon bond.

In more detail, the particulate pitch used is a high melting carbonpre-cursor and in these particular embodiments the pitch powder suppliedby RUTGERS VFT SA of Kekulestrasse3, D-44579 Castrop-Rauxel, Germanyunder the brand CARBORES is particularly suitable. The generalproperties of this powder are; glass transition temperature (TMA) 213°C., Coking value (ISO 6998) 85.5%, Benzo[a]pyrene{HPLC} 125 mg/kg.

The ceramic filler used is one or more of alumina, zirconia andmagnesia. Graphite powder is also added, of particule size generallyless than 2 mm.

Further additives may be included in the mix, including one or more ofsilicon carbide, anti-oxidants, metallic powders, glass frit,preservative material, stabilising material, anti-bacterial material,tabular alumina, mullite, silica, chromite, clay talc, mica, siliconnitride, zirconium diboride, titanium diboride, boron metal powder,magnesium metal powder, aluminium metal powder, boron carbide.

Water is added in sufficient quantities to produce a slurry, the slurrybeing mixed until the mixture is generally homogenous.

The slurry mix is then granulated by spray drying the mixture in aconventional spray drying tower. The granular particles formed are thencollected for pressing to form an intermediate product. An iso-staticpress is used to form the intermediate product. The intermediate productis then fired at temperatures in the order of 700° C. to 1000° C. forbetween 0.1 and 3 hours to cause graphitisable bonds to be formed toproduce the refractory article. Firing for one hour has been found toproduce satisfactory products. There is no requirement to temper theintermediate product, as is required in the conventional technique fortar pitch and phenolic resin. Preferably firing is carried out in theabsence of air or alternatively the intermediate product is glazed toprotect it from oxidation during pyrolysis.

This process has many other advantages over the aforementioned priorart. The nature of the process means that it can be continuous and doesnot require to be a batch process. The methodology could be fullyautomated to provide for continual production of refractory articles.

Further advantages are that the materials used, in particular theparticulate pitch is considerably less hazardous than conventional tarpitch or phenolic resin and the spray drying process is considerablymore controllable than the drying processes involved in conventionaltechniques. The granules also have a longer shelf life than conventionaltar pitch/phenolic resin mixes and the quality of the finished articlehas proven to be more consistent than that from the conventionaltechnique.

Graphitisable carbon bond generally has better oxidation resistance andsuperior thermal shock resistance than glassy carbon obtained fromphenolic resin.

The methodology of the present invention is also considerably quickerthan the conventional processes.

Below are examples of mixes that can be used in accordance with thepresent invention. 1. Alumina-Graphite mix Brown Fused alumina 0-80%Calcined alumina 0-80% Carbores 0-50% Silicon metal 0-50% Glass frit0-50% Clay materials 0-80% 2. Zirconia-Graphite mix Zirconia (ZrO²)0-80% Silicon carbide 0-30% Carbores 0-50% Silicon metal 0-50% Glassfrit 0-50% Clay materials 0-80% 3. Magnesia-Graphite mix MgO 0-80%Silicon carbide 0-30% Carbores 0-50% Silicon metal 0-50% Glass frit0-50% Clay materials 0-80%

The following are three specific examples of mixes which have been usedin accordance with the present invention to produce black refractoryarticles. Alumina-Graphite mix Brown Fused alumina 35% Calcined alumina20% Graphite 20% Carbores 5% Feldspar 4% Silicon metal 4% Glass frit 2%Zirconia-Graphite mix Fused Zirconia 79% Graphite 5% Carbores 4% Frit 1%Silicon metalpowder 4% Magnesia-Graphite mix Fused Magnesia powder 70%Graphite 13% Silicon metalpowder 4% Glass frit 2% Carbores 4%

Various modifications may be made to the above described embodimentwithout departing from the scope of the invention. CARBORES may bereplaced by one or more other pitches, and further may be used inconjunction with one or more other pitches.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

1-27. (canceled)
 28. A method for producing a refractory article, themethod comprising producing a mix of particulate pitch, ceramic fillerand water, spray drying the mix, forming the spray dried mix into anintermediate product and heating the intermediate product to produce arefractory article comprising graphitisable carbon bond.
 29. A method asclaimed in claim 28, in which the mix is produced in the form of aslurry.
 30. A method as claimed in claim 28, in which the mix isgenerally homogenous.
 31. A method as claimed in claim 28, in which theparticulate pitch used is in powder form.
 32. A method as claimed inclaim 28, in which the particulate pitch is a high melting carbon bondpre-cursor.
 33. A method as claimed in claim 28, in which the pitchcomprises CARBORES a material supplied by RUTGERS VFT AG ofCastrop-Rauxel, Germany.
 34. A method as claimed in claim 28, in whichthe ceramic filler used comprises one or more of alumina, zirconia,magnesia.
 35. A method as claimed in claim 28, in which the mix furthercomprises one or more of silicon carbide, anti-oxidant, metallic powder,glass frit, preservative material, stabilizing material, anti-bacterialmaterial, tabular alumina, mullite, silica, chromite, clay talc, mica,silicon carbide, silicon nitride, zirconium diboride, titanium diboride,boron metal powder, magnesium metal powder, aluminium metal powder,boron carbide.
 36. A method as claimed in claim 28, in which the mix isspray dried in spray drying apparatus.
 37. A method as claimed in claim36, in which the apparatus used is of conventional design.
 38. A methodas claimed in claim 36, in which the mix is spray dried to producegranules.
 39. A method as claimed in claim 36, in which the mix is spraydried to be free flowing.
 40. A method as claimed in claim 38, in whichthe granules are pressed.
 41. A method as claimed in claim 40, in whichthe granules are pressed at ambient temperature.
 42. A method as claimedin claim 40, in which the granules are pressed in an isostatic press.43. A method as claimed in claim 28, in which the intermediate productis heated to temperatures in the order of 700° C. to 1000° C., forbetween approximately 0.1 and 3 hours, to fire the product to producegraphitisable carbon bonds in the article.
 44. A method as claimed inclaim 28, in which heating is carried out generally in the absence ofair.
 45. A method as claimed in claim 28, in which the intermediateproduct is glazed to protect from oxidation during pyrolysis.
 46. Amethod as claimed in claim 28, in which the method is carried outcontinuously.
 47. A method as claimed in claim 28, in which the methodis automated.
 48. A method for producing granules for use in theproduction of refractory articles, the method comprising producing a mixof particulate pitch, ceramic filler and water and spray drying the mix.49. A refractory article produced by a method comprising producing a mixof particulate pitch, ceramic filler and water, spray drying the mix,forming the spray dried mix into an intermediate product and heating theintermediate product to produce a refractory article comprisinggraphitisable carbon bond.
 50. Granules for use in the production ofrefractory articles, the granules being produced by a method comprisingproducing a mix of particulate pitch, ceramic filler and water and spraydrying the mix.